Upgrade or Full Version of Win 7 Ultimate

I have a multi-boot system w/ Windows XP-64, Red Hat Enterprise Linux 5.3, and developers copy of Win 7 Ultimate provided by Microsoft for testing. It is a full version, but will go inactive soon. I've installed Windows 7 in a 300Gb partition but want to purchase the correct copy Upgrade or Full to continue using, testing, etc. My questions are:

Can I purchase the Upgrade and just apply the activation key for Win 7 Ultimate?

If I can't just apply the activation key , then will Upgrade allow me to install in just that partition? I want to maintain my XP-64 and RHEL. FYI, the original install of Win 7 absolutely ignored my RHEL boot record, fortunately Red Hat had a simple process to recover the boot record so I can still boot the OS I choose.

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